. Disseminated histoplasmosis occurs in a large percentage of AIDS patients who reside in highly endemic areas. The zoopathogenic fungus Histoplasma capsulatum requires iron for growth and like other pathogens must cope with the iron-limiting environment of the host. Since H. capsulatum is a facultative intracelllular parasite, it must rely on the iron reserves of the phagocyte. The long term goal of this work is to study the intracellular nutrition and metabolism of H. capsulatum for the purpose of understanding how intracellular growth of the fungus is interrupted in the macrophages of a recovering host. The goal of the work proposed in this grant is to study aspects of iron acquisition by H. capsulatum. There are three specific aims. (1) Hydroxymate siderophore production of the fungus will be monitored in vitro and following its infection in a murine macrophage cell line and in human monocytes. (2) Histoplasma membrane protein profiles will be revealed again in both in vitro grown organisms, incubated with and without iron sources, as well as following infection in the two populations of macrophages indicated in specific aim 1. The applicant will specifically look for protein alterations induced in the fungus by an iron-restrictive environment. Protein profiles will be examined by standard gel techniques and Western blotting. (3) Mutants of the fungus in hydroxymate siderophore production will be sought using restriction enzyme mediated integration (REMI), a technique similar in principal to transposon mutagenesis. The virulence of such mutants will be tested by their ability to grow in macrophages and by determining their virulence in a murine model of the infection. Such studies may lead to a more effective therapy in histoplasmosis by interrupting the supply of iron to the organism.